Dispersed catalysts are employed in slurry bed hydrocracking processes for converting heavy residues to transportation-fuel range fractions. Slurry hydrocracking is used for the primary upgrading of heavy hydrocarbon feed stocks obtained from the distillation of crude oil, including hydrocarbon residues or gas oils from atmospheric or vacuum distillation. In slurry hydrocracking, these liquid feed stocks are mixed with hydrogen and solid catalyst particles, e. g., as a particulate metallic compound such as a metal sulfide, to provide a slurry phase. Representative slurry hydrocracking reactors and processes are described, for example, in U.S. Pat. No. 5,755,955 and U.S. Pat. No. 5,474,977. Slurry hydrocracking produces naphtha, diesel, gas oil such as light VGO (LVGO) and heavy VGO (HVGO), and a low-value, refractory pitch stream. The VGO streams are typically further refined in catalytic hydrocracking or fluid catalytic cracking (FCC) to provide saleable products. To prevent and/or reduce the formation of coke precursors, the HVGO stream can be recycled to the slurry hydrocracking reactor.
If the level of solids in the slurry reactor exceeds certain levels, the solids will begin to collect at the bottom of the slurry reactor. This can lead to coking of the solids, plugging, reactor fouling, temperature anomalies, and other such issues. Additionally, removing the accumulated solids will require an undesirable shutdown of the slurry reactor. In order to control the accumulation of solids in the slurry reactor, a common practice is to periodically withdraw a drag stream from the bottom of the reactor. However, while removing a drag stream from the bottom of the reactor, the unconverted feed and product hydrocarbon might bypass and leave in the drag stream. The loss of the uncovered feed and the product hydrocarbon material is undesirable
In addition to unconverted feed and product materials, the drag stream contains organic and inorganic solids. The storage and disposal and of this stream could become a potential problem for the refiner.
The prior art regarding the strategies for the disposal of the drag stream might involve storing this stream in a slop tank and burning it as a fuel in cement kilns. If dissolved gases, and naphtha and distillate range hydrocarbons are removed, the drag stream may contain about 30 wt % VGO, 50 wt % material with an initial boiling point above 524° C. (975° F.), and 20 wt % solids.
Therefore, there remains a need for an effective and efficient process for recovering the hydrocarbons from a drag stream from a slurry hydrocracking unit.